Wireless communication networks are widely known in which base stations (BSs) communicate with user equipments (UEs) (also called terminals, or subscriber or mobile stations) within range of the BSs.
The geographical area covered by one or more base stations is generally referred to as a cell, and typically many BSs are provided in appropriate locations so as to form a network or system covering a wide geographical area more or less seamlessly with adjacent and/or overlapping cells. (In this specification, the terms “system” and “network” are used synonymously except where the context requires otherwise). Each BS divides its available bandwidth into individual resource allocations for the user equipments which it serves. The user equipments are generally mobile and therefore may move among the cells, prompting a need for handovers between the base stations of adjacent and/or overlapping cells. A user equipment may be in range of (i.e. able to detect signals from) several cells at the same time, and it is possible for one cell to be wholly contained within a larger cell, but in the simplest case the UE communicates with one “serving” cell.
The direction of communication from the base station to the UE is referred to as the downlink (DL), and that from the UE to the base station as the uplink (UL). Two well-known transmission modes for a wireless communication system are TDD (Time Division Duplexing), in which downlink and uplink transmissions occur on the same carrier frequency and are separated in time, and FDD (Frequency Division Duplexing) in which transmission occurs simultaneously on DL and UL using different carrier frequencies.
Resources in such a system have both a time dimension and a frequency dimension. In LTE, the resources in the time domain are organised in units of frames, each having a plurality of “subframes”. Frames follow successively one immediately after the other. An FDD frame consists of 10 uplink subframes and 10 downlink subframes occurring simultaneously. An FDD frame has duration of 10 ms and each subframe a duration of 1 ms. In TDD, the 10 subframes are shared between UL and DL and various allocations of subframes to downlink and uplink are possible, depending on the load conditions. Subframes may consequently be referred to as uplink subframes or downlink subframes.
The basic system architecture in LTE is illustrated in FIG. 1. As can be seen, each UE 20 connects over a wireless link via a Uu interface to an eNodeB (eNB) 10, which defines one or a number of cells for wireless communication.
Each eNB 10 in turn is connected by a (usually) wired link using an interface called S1 to higher-level or “core network” entities, including a Serving Gateway (S-GW 22), and a Mobility Management Entity (MME 21) for managing the network and sending control signalling to other nodes, particularly eNBs, in the network. These are shown in combined form in the Figure. The links to the core network are referred to as “backhaul”. The backhaul is defined as the transport network which allows connecting all network nodes together, namely each eNB to the core network entities and each eNB to its neighbour eNB if needed. Generally, backhaul is based on Internet Protocol (IP), and for femto stations only (HNBs and HeNBs), backhaul may be over existing broadband infrastructure in homes and offices
In FIG. 1, the S1 interface is labelled S1-U, the suffix U denoting the user plane employed by the eNBs 10 for communicating user data to and from the S-GW 22. The S-GW is responsible for packet forwarding of user data on the downlink to the UE 20 and on the uplink. The S-GW 22 provides a “mobility anchor” for the user plane during handovers of a UE 20 from one eNB 10 to another.
In parallel to this, there is an interface S1-MME (sometimes called S1C) via which the eNBs 10 exchange control messages with the MME 21. The main function of the MME 21, as its name suggests, is to manage mobility of the UEs 20, and it is a signalling-only entity. The MME 21 is also responsible for controlling security (including authenticating users). In practice, there may be several MMEs forming a MME “pool”. One eNB can have several S1-MME interfaces towards several MMEs.
In addition, as shown in FIG. 1, the eNBs 10 communicate among themselves by a (usually) wireless link, using an interface called X2 for mutual co-ordination, for example when handing over a UE 20 from one eNB to another. There is only one X2 interface between two eNBs. The application layer signalling protocol is referred to as X2AP.
Several “channels” for data and signalling are defined at various levels of abstraction within the network. FIG. 2 shows some of the channels defined in LTE at each of a logical level, transport layer level and physical layer level, and the mappings between them.
At the physical layer level, on the downlink, user data as well as System Information Blocks (SIBs) are contained in a transport channel DL-SCH, carried on the Physical Downlink Shared Channel (PDSCH). As can be seen from FIG. 2, PDSCH also carries a paging channel PCH at the transport layer level. There are various control channels on the downlink, which carry signalling for various purposes; in particular the Physical Downlink Control Channel, PDCCH, is used to carry, for example, scheduling information from a base station (called eNB in LTE) to individual UEs being served by that base station. The PDCCH is located in the first OFDM symbols of a slot.
Each base station broadcasts a number of channels and signals to all UEs within range, whether or not the UE is currently being served by that cell. Of particular interest for present purposes, these include a Physical Broadcast Channel PBCH as shown in FIG. 3, as well as (not shown) a Primary Synchronization Signal PSS and Secondary Synchronization Signal SSS. PBCH carries a so-called Master Information Block (MIB), which gives, to any UEs within range of the signal, basic information including system bandwidth, number of transmit antenna ports, and system frame number. Reading the MIB enables the UE to receive and decode the SIBs referred to earlier.
Meanwhile, on the uplink, user data and also some signalling data is carried on the Physical Uplink Shared Channel (PUSCH), and control channels include a Physical Uplink Control Channel, PUCCH, used to carry signalling from UEs including channel quality indication (CQI) reports and scheduling requests.
The Physical Random Access Channel PRACH is used to carry the Random Access Channel (RACH) for accessing the network if the UE does not have any allocated uplink transmission resource. If a scheduling request (SR) is triggered at the UE, for example by arrival of data for transmission on PUSCH, when no PUSCH resources have been allocated to the UE, the SR is transmitted on a dedicated resource for this purpose. If no such resources have been allocated to the UE, the RA procedure is initiated. The transmission of SR is effectively a request for uplink radio resource on the PUSCH for data transmission.
Thus, RACH is provided to enable UEs to transmit signals in the uplink without having any dedicated resources available, such that more than one terminal can transmit in the same PRACH resources simultaneously. The term “Random Access” (RA) is used because (except in the case of contention-free RACH, described below) the identity of the UE (or UEs) using the resources at any given time is not known in advance by the network. Preambles (which when transmitted, produce a signal with a signatures which can be identified by the eNB) are employed by the UEs to allow the eNB to distinguish between different sources of transmission.
Situations where the RACH process is used include:                Initial access from RRC_IDLE        RRC connection re-establishment        Handover        DL data arrival in RRC_CONNECTED (when non-synchronised)        UL data arrival in RRC_CONNECTED (when non-synchronised, or no SR resources are available)        Positioning (based on Timing Advance)        
RACH can be used by the UEs in either of contention-based and contention-free modes. In contention-based RA, UEs select any preamble at random, at the risk of “collision” at the eNB if two or more UEs accidentally select the same preamble. Contention-free RA avoids collision by the eNB informing each UE which preambles may be used.
The RA procedure can be triggered in response to a PDCCH order (e.g. for DL data arrival, or positioning). Contention free RA in Release-8/9/10 is only applicable for handover, DL data arrival and positioning.
Referring to FIG. 3, the RA procedure typically operates as follows (for contention based access):—
(i) As already mentioned the UE 20 receives the downlink broadcast channel PBCH for the cell of interest (serving cell). This is indicated by (0) in the Figure.
(ii) The network, represented in FIG. 3 by eNB 10, indicates cell specific information including the following:                resources available for PRACH        Random Access Preambles (henceforth, “preambles”) available (up to 64)        preambles corresponding to small and large message sizes.        
(iii) The UE 20 selects a PRACH preamble according to those available for contention based access and the intended message size.
(iv) The UE 20 transmits the PRACH preamble (also called “Message 1”, indicated by (1) in the Figure) on the uplink of the serving cell. The network (more particularly the eNB 20 of the serving cell) receives Message 1 and estimates the transmission timing of the UE.
(v) The UE 20 monitors a specified downlink channel for a response from the network (in other words from the eNB 10). The UE monitors this channel for a specified length of time, the RAR window shown in the Figure. In response to the UE's transmission of Message 1, the UE receives a Random Access Response or RAR (“Message 2” indicated by (2) in FIG. 3) from the network. This is described in more detail below.
(vi) In response to receiving Message 2 from the network, the UE 20 makes a scheduled transmission on PUSCH (“Message 3” as indicated by (3) in the Figure) using the UL grant and TA information contained in Message 2.
(vii) A contention resolution message may be sent from the network (in this case from the eNB 10) in the event that the eNB received the same preamble simultaneously from more than one UE, and more than one of these UEs transmitted Message 3.
If the UE 20 does not receive any response from the eNB, the UE selects a new preamble and sends a new transmission in a RACH subframe after a random back-off time.
For contention-free RA, the procedure is simpler:
(i) The eNB configures the UE with a preamble from those available for contention-free access.
(ii) The UE transmits the preamble (Message 1) on the uplink of the serving cell.
(iii) The UE receives the RAR (Message 2) via PDSCH from the network, which contains an UL grant for transmission on PUSCH. The resource to be used for RAR is again signalled on PDCCH using CSS.
In both contention-based and contention-free RA procedures, the RAR contains a Temporary Cell Radio Network Temporary Identifier (C-RNTI) which identifies the UE. The C-RNTI is used to address UL transmission to the cell by identifying the UE in later UL transmissions.
In the contention-based procedure, the UE transmits this C-RNTI back to the eNB in Message 3 and, if more than one UE does so there will be a collision at the eNB which may then initiate the contention resolution procedure.
In both contention-based and contention-free RA procedures, the RAR contains:                A Timing Advance command;        A UL Grant; and        A Temporary Cell Radio Network Temporary Identifier (C-RNTI).        
At the physical layer, the 20-bit UL Grant is interpreted as the Random Access Response Grant (RARG), and contains amongst others things an indication of the UL resources and transmit power control (TPC) to be used for the forthcoming transmission on PUSCH.
The various fields of the RAR and RARG may or may not be relevant to a given RA procedure, depending on whether it is contention-based or contention-free, and depending on its purpose.
The Temporary C-RNTI becomes permanent after contention-resolution and further attempts in the contention-based RA procedure.
In order to manage, e.g., scheduling and congestion issues in the downlink, the eNB is given a window of time to send the RAR. This is the RAR window mentioned earlier and shown in FIG. 3, and is configurable from two to ten milliseconds (corresponding to two to ten subframes), depending upon the choice of system parameters. The UE is required to monitor the whole RAR window to detect if the eNB has acknowledged its preamble transmission. If no RAR is received within the RAR window then typically the RA preamble is re-transmitted.
Further details of the RA procedure in LTE are given in the document 3GPP TS 36.321, hereby incorporated by reference.
As already mentioned, cells may be overlapping or even entirely contained within a larger cell. This is particularly the case for so-called Heterogeneous Networks.
FIG. 4 schematically illustrates part of a heterogeneous network in which a macro base station 10 covers a macrocell area (indicated by the large outer ellipse), within which there are other, overlapping cells (the smaller ellipses) formed by a pico base station 12 (picocell) and femto base stations 14. As shown a UE 20 may be in communication with one or more cells simultaneously, in this example with the macro cell and the picocell. The cells may not have the same bandwidth; typically, the macro cell will have a wider bandwidth than each pico/femto cell.
Some definitions are as follows:                Heterogeneous Network: A deployment that supports a mixture of more than one of macro, pico, femto stations and/or relays in the same spectrum.        Macro base station—conventional base stations that use dedicated backhaul and open to public access. Typical transmit power ˜43 dBm; antenna gain ˜12-15 dBi.        Pico base station—low power base station with dedicated backhaul connection and open to public access. Typical transmit power range from ˜23 dBm-30 dBm, 0-5 dBi antenna gain;        Femto base station—consumer-deployable base stations that utilize consumer's broadband connection as backhaul; femto base stations may have restricted association. Typical transmit power <23 dBm.        Relays—base stations using the same radio spectrum for backhaul and access. Similar power to a Pico base station.        
In LTE, an example of a femto base station is the so-called Home eNodeB or HeNB.
The installation by network customers of base stations with a localised network coverage cell, such as femto base stations (Home eNodeBs) is expected to become widespread in future LTE deployments. A femto base station or pico base station can be installed in, for example, a building within which network subscriber stations experience high path loss in transmissions with a macro cell. Femto and pico base stations can be installed by a customer in his own premises. The femto and picocells thereby formed can improve network coverage, but for coordination among the various cells, it is preferable that they be able to exchange information with the macro cell (more precisely the MeNB 10 of FIG. 4), in some cases with other cells, and that they are synchronized with one another.
Pico eNBs and femto eNBs differ in that a pico eNB supports the X2 interface with a macro eNB, whilst a femto cell (HeNB), in current LTE specifications, does not always support the X2 interface with a macro eNB. However, since a femto cell has an S1 interface to the core network, it would be possible in principle for it to receive such exchanges over S1. The term “picocell” will henceforth be used to cover both femto and picocells, and “peNB” used to cover both a pico and femto base station.
Consider now a heterogeneous network (HetNet) in which UEs operate within the coverage of at least two cells: a macrocell to which they are presently connected and a picocell to which they may cause uplink (UL) interference. The picocell eNodeB (peNB) and macrocell eNodeB (MeNB) are able to exchange information (normally over the X2 interface), and the macrocell is able to effect handover of any macro-connected UE (MUE) to the picocell, making it a pico-connected UE (pUE).
The peNB may need to estimate the UL interference MUEs are causing so that it can provide load information to the MeNB in order to facilitate, e.g. re-scheduling of UL transmission in the macrocell or handover of certain MUEs to the picocell whence they will no longer cause interference in the picocell. It is preferred that solutions to this should not rely on tight synchronization among the layers of the network. Therefore, on the uplink, using the Random Access (RA) procedure and measuring the transmission on the Physical Random Access Channel (PRACH) are a useful solution.
It has been proposed in the above scenario that the MeNB triggers, via a PDCCH order, a contention-free RA procedure from a UE. The peNB is informed over X2 of the resources and preamble the UE will use, and the peNB can then measure the received signal and use this to estimate the interference the UE will cause when transmitting, e.g., PUSCH and PUCCH.
Following this proposal, the UE will expect the contention-free RA procedure to run to full completion, including, for example, re-transmission of the preamble or the RAR if the procedure does not complete successfully the first time. However, the peNB may be able to obtain satisfactory interference estimates without the procedure being fully complete, and there could thus be wasted transmit power, signalling, and processing effort at both the UE and eNBs, which could additionally disrupt conventional RA procedures in the macrocell by occupying resources. Furthermore, the RA procedure in LTE allows for power ramping if the preamble is not received successfully at the eNB. This is undesirable in the scenario considered here since ideally the transmission on PRACH would have a power approximately the same as typical transmission on PUSCH. Additionally, power-ramping would make it difficult to form a meaningful time-average of the interference at the peNB. Trying to use the X2 interface from the peNB to MeNB to trigger, terminate, configure, etc. a contention-free RA procedure in the macrocell according to the peNB's interference measurement needs will be too slow compared to the several millisecond timescale of the RA procedure at lower layers.
Therefore, schemes that minimise wasted procedures and associated signalling, whilst enabling suitable interference measurements at the peNB to be configured in a timely manner, are of significant interest.